This application claims priority to Japanese Patent Application No. 2000-176850 filed Jun. 13, 2000, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to so-called tandem-type color image forming apparatus.
2. Description of the Related Art
Color image forming apparatus of the so-called tandem type are known from, for example, Japanese Patent Laid-Open Gazette No. HEI 7-28294, in which a plurality of image forming units containing respective color toners are disposed along a transfer-receiving medium.
In image forming apparatus of this type, images each colored different than another formed by respective image forming units are primarily transferred to an intermediate transfer belt as superimposed one upon another by means of a transfer member such as a transfer roller and the images thus superimposed are then secondarily transferred as a single image to a paper sheet from the intermediate transfer belt by means of a transfer member. Alternatively, the images formed by respective image forming units are transferred to a paper sheet being fed by a feeding transfer belt by means of a transfer member. The image on the paper sheet is fused there to by heating during the passage through a fusion unit and then ejected to an ejected paper receiving tray.
In such a prior art tandem-type image forming apparatus, control over the transfer power, specifically, control over the transfer current or the transfer voltage, for example, Active Transfer Voltage Control (ATVC) is performed to maintain the transfer performance constant notwithstanding any change in environmental conditions or the like. The ATVC is such that a voltage to be applied to a transfer member when any image is not formed is controlled so that a current flowing through the transfer member assumes a predetermined constant value and a transfer voltage to be applied to the transfer member when an image is formed is determined based on the voltage applied when any image is not formed. This transfer voltage is controlled to be constant during the image forming operation. The ATVC ensures constant transfer performance thereby to allow stabilized images to be formed.
In such a prior art tandem-type color image forming apparatus, however, the control for determining a transfer current for a primary transfer roller is performed separately from that for a secondary transfer roller and, hence, it takes a long time to complete the controls for determining all the necessary transfer currents. Particularly where the control for determining a transfer current is performed during a warm-up time after the power supply of the image forming apparatus has been turned on, a longer warm-up time must be secured.
A chief object of the present invention is to provide a tandem-type image forming apparatus which requires a shorter period of time for completing controls for determining transfer powers.
Another object of the present invention is to provide a tandem-type image forming apparatus which is capable of ensuring constant transfer performance thereby to form stabilized images.
These and other objects can be attained by an image forming apparatus comprising: rotatable transfer-receiving medium; a plurality of image forming devices disposed in row along the transfer-receiving medium for forming respective images each colored different than another; transfer members each disposed at a location opposite to each of the image forming devices across the transfer-receiving medium and applied with respective transfer powers for causing the images formed by respective image forming devices to transfer to the transfer-receiving medium; and a controller configured to simultaneously perform controls for determining the transfer powers to be applied to those transfer members which are not located adjacent to each other while performing controls for determining the transfer powers to be applied to those transfer members which are located adjacent to each other at separate times.
Particularly where an image forming apparatus is of the type wherein images primarily transferred to the transfer-receiving medium are secondarily transferred to a paper sheet, the aforementioned objects can be attained by an image forming apparatus comprising: rotatable transfer-receiving medium; a plurality of image forming devices disposed in row along the transfer-receiving medium for forming respective images each colored different than another; primary transfer members each disposed at a location opposite to each of the image forming devices across the transfer-receiving medium and applied with primary transfer powers for causing the images formed by respective image forming devices to primarily transfer to the transfer-receiving medium; a secondary transfer member disposed in contact with the transfer-receiving medium and applied with a secondary transfer power for causing the images primarily transferred to the transfer-receiving medium to secondarily transfer to a paper sheet passing through the contact portion thereof; and a controller configured to simultaneously perform controls for determining the transfer powers to be applied to those transfer members which are not located adjacent to each other while performing controls for determining the transfer powers to be applied to those transfer members which are located adjacent to each other at separate times.
Such image forming apparatus are adapted to simultaneously perform controls for determining the transfer powers to be applied to those transfer members which are not located adjacent to each other and hence are capable of completing the determination of the transfer powers in a shorter time than the case where controls for sequentially determining the transfer powers for every transfer member are performed. Accordingly, when such controls are performed during the warm-up time after the power has been turned on, the time required for warm-up can be shortened. Further, the image forming apparatus are adapted to perform controls for determining the transfer powers to be applied to those transfer members which are located adjacent to each other at separate times and hence are capable of determining the transfer powers with higher precision.
The image forming apparatus described above may be configured such that the controls for simultaneously determining the primary transfer powers to be applied to first and third ones of the transfer members as counted from the upstream side in a moving direction of the transfer-receiving medium are performed separately from the controls for simultaneously determining the primary transfer powers to second and fourth ones of the transfer members as counted from the upstream side in the moving direction of the transfer-receiving medium.
Also, it is possible that the control for determining the secondary transfer power is performed simultaneously with the controls for determining the primary transfer powers to be applied to the primary transfer members.
Such an arrangement may be employed that the primary transfer members and the secondary transfer members may be identically-shaped members constructed of a same material and the secondary transfer power is determined based on the primary transfer powers having been determined.
Preferably, an automatic density control and a registration correction are performed after the primary transfer powers and the second transfer power have been determined.
The transfer-receiving medium may be an intermediate transfer belt, a transfer conveyor belt, or a like member.
The aforementioned objects of the present invention can also be attained by an image forming apparatus comprising: rotatable transfer-receiving medium; a plurality of image forming devices disposed in row along the transfer-receiving medium for forming respective images each colored different than another; primary transfer members each disposed at a location opposite to each of the image forming devices across the transfer-receiving medium and applied with respective primary transfer powers for causing the images formed by respective image forming devices to primarily transfer to the transfer-receiving medium; a secondary transfer member disposed in contact with the transfer-receiving medium and applied with a secondary transfer power for causing the images primarily transferred to the transfer-receiving medium to secondarily transfer to a paper sheet passing through the contact portion thereof; and a controller configured to first perform a control for determining the secondary transfer power and then perform controls for determining the primary transfer powers based on the secondary transfer power having been determined.